This invention relates to a novel control system for d-c feeder breakers employed in a transit type system, and more specifically relates to a novel reclosing system which permits the closing of a d-c breaker only after determining that there is no short circuit on the feeder circuit containing the circuit breaker by alternately measuring the voltage on the feeder circuit and, if no voltage appears, the voltage drop produced by a load-measuring current injected into the feeder circuit.
There are many known applications which require the use of a d-c circuit breaker which can be closed only after it is determined that the circuit containing the circuit breaker is free of a fault. Typical of such systems is a transit system in which electrically powered vehicles traveling on energized rails draw power from one or more circuits and in which the cars themselves contain chopper type voltage regulators. The feeder circuits in these transit systems are protected by d-c breakers which must not be permitted to close if there is a fault on the feeder circuit. Such systems have used control circuits which measure the feeder circuit voltage before permitting closure of the breaker. Thus, if feeder circuit voltage exists, it is indicative that the feeder circuit is free of fault and the d-c feeder breaker can be closed.
In many transit systems, such as a radial type of electrical system or in a network system having an open substation breaker at the remote end of the line, it is possible that a feeder breaker is open and the feeder circuit will not have any voltage thereacross even though the circuit is free of a fault. In this situation, it is known that a current can be caused to flow in the feeder circuit and the circuit resistance can be measured to determine whether or not a fault exists. If no fault is found, then the feeder breaker can be closed.
The prior art control circuits have been subject to numerous disadvantages. Among these disadvantages are:
(a) They require a large number of components.
(b) The individual components are complex and difficult to calibrate.
(c) The systems had relatively large space requirements and were relatively expensive.
(d) The systems have been susceptible to allowing incorrect closing of the feeder breaker due to negative track voltage resulting from cross-bonded running rails.
(e) The systems have not been useful in relatively high voltage systems up to about 1000 volts d-c.